step involved in catalytic reaction - copy

7/25/2019 Step Involved in Catalytic Reaction - Copy

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Step Involved in

Catalytic Reaction


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1. Mass transfer (diusion of the reactants) (e.g.. species A) fro

"uid to the e#ternal surface of the catalyst pellet.

$. %iusion of the reactant from the pore mouth through the ca

to the intermediate vicinity of the internal catalytic surface.

&. Adsorption of reactant A onto the catalyst surface.

'. Reaction on the surface of the catalyst (e.g. A )

. %esorption of the products (e.g. ) from the surface.

*. %iusion of the products from the interior of the pellet to the

at the e#ternal surface.

+. Mass transfer of the products from the e#ternal pellet surface

"uid.


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Steps in heterogeneous catalytic rea

Reaction ta!esplace on thesurface, ut thespecies involved inreaction must get toand from thesurface


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-he overall rate of reaction is eual to the rate of the

step in the mechanism.

0hen the difusion steps (1,$,*, and +) are very a

compared /ith the reaction steps (&, '. and ), the

concentrations in the immediate vicinity of the active

indistinguishablefrom those in the ul! "uid. In thi

the transport or difusion steps do not afectthe o

of the reaction.

Reaction steps are very ast compared /ith the di

steps, mass transport does afectthe reaction rate


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Step I vervie/2 %iusion from thul! to the 3#ternal -ransport

ul! concentration CAmust travel through the oundary layer o

thic!ness 4 to the e#ternal surface of the pellet /here the concenis CASas sho/n in 5igure. Rate of transfer2

6c2 mass transfer coe7cient

Rate


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Low Velocitiesoundary layer is thick9ong time for A to travel to the surfaceSmall mass transfer coe7cient !c.

Mass transfer across the oundary layer is slo/

and limits the rate of the overall reaction.

Very high velocitiesoundary layer is so smoers any resistance toacross the oundary la3#ternal mass transfer

rate of reaction.


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3#ternal resistance also decreases as the particle si:e is d

As the "uid velocity increases and;or the particle diametethe mass transfer coe7cient increases until a plateau is r


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Step $ vervie/2 Internal %iusio

As Adiuses into the interior of the pellet, it reacts /ith catalysthe sides of pore /alls.

5or large pellets, it ta!es a long time for the reactant A to diuscompared to the time it ta!es for the reaction to occur on the insurface.


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A model system is proposed and then the isotherm ota

from the model is compared /ith the e#perimental data

on the curve.

If the curve predicted y the model agrees /ith the

e#perimental one, the model may reasonaly descrie /

occurring physically in the real system.

If the predicted curve does not agree /ith data otained

e#perimentally, the model fails to match the physical sit

at least one important characteristic and perhaps more.


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Adsorption Isotherms

Isotherms portray the amount of a gas adsored n a solidpressures ut at one temperature.

A physical phenomena


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Adsorption Isotherms

S 2 an active site= alone it /ill denote a vacant site,

-he comination of S /ith another 9etter (e.g., A.S) /ill meunit of species A /ill e adsored on the site S.

-he adsorption of A on a site S is represented y

total molar concentration of active per unit mass of catalyst is eual tonumer of active sites per unit masdivided by Avogadro's numbera

e laelled C, (mol;g .cat).


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Asence of catalyst deactivation2 total concentration of actremains constant.

?i partial pressure of species i in the gas phase. atm or !?

Ci.s surface concentration of sites occupied y species i, rno

A conceptual mode depicting species A and ! on two

5or the system sho/n, the total concentration of sites is

"it


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Adsorption of caron mono#idemetal surfaces

molecular or non#dissociated adsorption

dissociat

C @ S C @$S


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Adsorption o carbon mono$idemolecules

C @ S

rate o attachment of the caronmono#ide molecules to the activesite on the surface

proportionalnumber o comolecules maactive site pe

a specic fraction of the molecules t

stri!e the surface ecome adsored Caron mono#ide partial pressure, ?co Concentration of vacant sites, Cv


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%etachment of caron mono#ide molecules from the surface isdirectly proportional to the concentration of sites occupied y thadsored molecules

-he netrate o adsorption is eual to the rate of molecular atsurface minus the rate of detachment from the surface.

B

adsorption

con

B ) (

(


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ecause caron mono#ide is the only material adsored on thecatalyst the site alance gives

(

At euilirium, the net rate of adsorption euals to :ero from eu

(

5rom euation *

Rearranging

adsorpt

9angm


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9angmuirBDinshel/ood (9D) mechani

-he adsored species are held onto denite points of attac

surface. 3ach site can accommodate only one adsored sp

-he surface is completely uniform so that there is the sam

of adsorption on all sites. Attractive or repulsive forces et

adEacent adsored molecules are ta!en to e negligile.


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Amount of C adsored per unit mass of catalyfunction of the partial pressure of C


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Isotherm for caron mono#ideadsored as atoms (

)

%issociative ads

C @$S

5or a molecule to dissociate as it adsored, t/o adEacent vacaactive sites are reuired rather than the sing site needed /hesustance adsors in its molecular form

?roaility of t/o vacant sites occurring adEacent to one anotis proportional to the suare of the concentration of vacant si


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5or desorption to occur, t/o occupied sites must e adEacen

B

5actoring out !A, the euation for dissociative adsorption

B )

9anisotadsato

?lease /ritedo/n thesteps FFF


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0hen more than one sustance present

Dome AssignmentFFFFFF


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"uraceReaction-he rate of adsorption of species A onto a solid surface,

A @ S

B )

After a reactant has een adsored onto the surface, it is caof reacting in a numer of /ays to form the reaction produc


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Reaction &echanism

"ingle site2 only the site on /hich the reactant is adso

involved

%ual site2 adsored reactant interacts /ith another sit

reaction et/een two adsorbed species

reaction of t/o species adsored on diferen

sites

ley#Rideal( the reaction et/een an adsored molecu

molecule in the gas phase


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"ingle site

only the site on /hich the reactant is adsored is involve

an adsored molecule of A may isomeri:e (or perhaps deco

directly on the site to /hich it is attached.

A.S

ecause in each step the reaction mechanism is elementary, surface reaction rate la/ is

B )


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%ual site

adsored reactant interacts /ith another site (either unoc

occupied) to form the product

A.S @ S

B )

corresponding surface reaction rate la/

dehydration of butanol


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%ual site

Reaction et/een t/o adsored species

A.S @ .S

surface reaction rate la/

B )


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the reaction of t/o species adsored on dierent types of

%ual site

A.SG @ .S


B )


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the reaction et/een an adsored molecule and a molecuphase.

ley#Rideal

A.S @ (g)

B )


propylene and en:ene


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%esorption

the products of the surface reaction adsored on the surfasuseuently desored into the gas phase

C . S

-he rate of desorption ofC

B )

6%Cis the desorption euilirium constant


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-he RateB9imiting Step

0hen heterogeneous reactions are carried out at steady stof each of the three reaction steps in series (adsorption, suand desorption) are eual to one another.

ne particular step in the series is usually found to e rarerateBcontrolling.

if /e could ma!e this particular step go faster, the entire rproceed at an accelerated rate.

In /riting this seuence. one must choose amomechanisms as molecular or atomic adsorptionor dualBsite reaction. He#t, rate Ia/s are /ritten

individual steps as sho/n in the preceding sect


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synthesis of ammonia from hydand nitrogen (adsorptionBlimited)

Rapid

Rapid

Rate 9


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-/o no#ious automoile e#haustproducts C and H(SurfaceBlimiting )

Rapid

Rapid

Rate9imiti


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Synthesi:ing a Rate 9a/, Mechand RateB9imiting Step

decomposition of cumene to form en:ene and propylene


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Steps in 9angmuirBDinshel/ood6inetic Mechanism


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Rate of adsorption of cumene

Surface reaction2 Adsored en:ene and propylene in gas pha

rate of en:ene desorptian


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%esorption is Eust the reverse of the adsorption ofen:ene

Rate of desorption may e /ritten as2

ecause there is no accumulation of reacting species on the

surface


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y assuming that this (or any other) srateBlimiting, /e are considering that

reaction rate constant of this step (in


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Is the Adsorption of Cumene Rate9imitingF

5or adsorption limited reactions, !Ais small and !Sand !%are

Conseuently, the ratios rS;!Sand r%;!%are very small (appro#

:ero), /hereas he ratio rA%;!Ais relatively large.

)eed to e$press *V+ and **+"in terms o ,*+ ,!and


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Surface reaction rate la/ (&

5or adsorptionBlimited reactions the surface specic reaction rat

large y comparison

(&$)(&

5or adsorptionBlimited reactions, !%is large y comparison

(&') (

(y (&&) (&)


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Replacing CC.Sin the rate euation y 3uation (&*) and then facto

y setting rA%K L, the term (6S6C;6) is simply the overall partial p

euilirium constant, 6?for the reaction


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Cv, can e eliminated from 3uation (&+) y utili:ing the site alan


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Cumene reaction rate la/ if adsorption /ere the limitingstep

Ho propresen

??K ?rate is


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Is the "urace Reaction Rate#Lim

rate of surface reaction

kAand kD, are very large y comparison /ith !S

rA%;!AK L K r%;!% and site alance

Initial rate


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Initial rate

At lo/ pressure

At highpressure


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Is the desorption Rate#Limiting-


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Actual initial rate as a function of partial pressure of cumene

Surface limited reaction mechanism is consistene#perimental data


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If /e /ere to have an adsoring inert in the feed

Inert /ould not participate in the reaction ut /ould occupythe catalyst surface

Rate la/ 5or surface reaction control /hen an adsoring ine

step involved in catalytic reaction - copy

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